The invention is in the field of nuclear magnetic resonance apparatus and methods and pertains especially to structure for RF coupling to a discrete sample.
In the apparatus and practice of NMR measurements, particular constraints are experienced for the probe. This portion of the apparatus couples RF energy from a transmitter-modulator to a sample and from the sample to the RF receiver. The general probe structure may house a number of other functions, such as sample spinning, temperature control apparatus, RF shielding and the like. This structure is subject to constraints related to magnetic properties of the components therein, but spatial limitations remain the most critical together with optimum coupling.
For discrete samples, whether for imaging or analytic studies, the RF coupling to the sample is achieved with a resonator surrounding the sample. The resonant structure, or insert, has taken the form of an LC circuit, the inductance being furnished by a helical coil, or most commonly, as a saddle coil or birdcage resonator. Capacitance is furnished in the form of discrete chip capacitors and/or the distributed self capacitance of the inductive member. Saddle coils are most often implemented at high frequencies (proton resonances at greater than 4.6 Tesla) as a Helmholtz pair of single (or multiple) turn loops in parallel, or series, with the sample disposed therebetween. The single turn loops are most often defined on a planar conductor and then rolled into cylindrical form which may be supported by a suitable frame or may be cantilevered to surround and define the volume portion to be occupied by the sample. Self-supporting coil structures may also be formed from wire.
In addition to the conducting paths forming the saddle coil (or other resonator) there is often other related conducting structure in close proximity to the coil structure. For example, floating or non-floating RF shields are often employed to define and limit the interior volume in respect of the RF field distribution. These ordinarily take the form of cylindrical portions or closure disks coaxially mounted in respect of the coil. These floating shields are known to contribute capacitance to the proximate resonant circuit. These coaxial structures have also been implemented outside the RF coil axially spaced from the window of the RF saddle coil. Examples of such structures are found in Varian, Inc. NMR probe designated 1H{13C/31P} Triple.nmr(trademark), and also as described in U.S. Pat. No. 5,192,911 and 5,262,727, commonly assigned herewith.
In order to furnish sufficient capacitance to the LC circuit, the prior art employed an electrically floating conducting cylinder in the manner described by the above referenced U.S. Pat. No. 5,192,911 patent, often disposed surrounding and spaced from a portion of the saddle coil. This outer cylinder or band provides a distributed capacitance in combination with that proximate portion of the saddle coil. In use, this prior art outer cylinder is axially disposed away from the edge of the xe2x80x9cwindowxe2x80x9d formed by the inner bound of the loop of the saddle coil turn to avoid field distortion and at least partially for that reason the prior art is limited to a relatively small ring about the leg, or terminal region of the coil.
The present invention employs a capacitor band which matches, to a selected extent, that portion of the projected surface shape of the inductor of the resonant circuit of the probe which contain the major contribution to the RF current density. The capacitor band of the invention includes a slot to avoid completed current loops to reduce losses from inductive coupling between the coil and the capacitor band, and to distribute the image currents on the capacitor band to correspond to the real RF current distribution on the inductor.